Just bought a new unit... Upgrade fw or not?

There are indeed a lot of bug reports, but I wouldn't worry too much because no user sees all the reported issues. For instance, I have no issues with heart rate (OHR, HRM-Pro Plus, TickrX) or Bluetooth audio connectivity (Momentum TW3), yet some users clearly do.

The issues I see are mostly issues unresolved from release (also on FW 10.10): 'Track Run' is no more accurate than 'Run' (no calibration file created), some activity transition jerkiness & minor bugs, VO2 max & training status being unresponsive to performance changes... Nothing is keeping me from heading outside, recording activities successfully, day in and day out.

Bottom line is there are no guarantees: something you care about might break and I think Garmin should work on a downgrade path to avoid headaches and anxiousness. But upgrading is unlikely to be the dumpster fire skimming through these threads might give you the impression of; people don't start threads to highlight stuff not breaking.

Convel said:

Nothing is keeping me from heading outside, recording activities successfully, day in and day out.

Nothing besides the battery being dead due to sync bugs, the HR sensor cadence locking all the time, the GPS and distance accuracy being off and a few other minor issues.

Not in mine. Maybe you are not able to use properly your fr955

Would you mind sharing all your current watch settings along with your usage patterns? 

The biggest settings that matter for battery life IMO are:

1) leaving Bluetooth off all the time except for occasional manual syncs

2) leaving wifi off all the time except for manual syncs

3) leaving pulse oximeter off almost all the time (set it to when sleeping only)

4) Turn your activity tracking alerts all off

5) having a watch face without seconds or any other attributes that update constantly.  I intentionally don't use pulse on my main watch face screen because it nearly doubles my battery drain.  Some 3rd party watch faces can also just be poorly coded and inefficient... So be careful what you use. 

6) use auto or gps only for tracking your training activities in situations where you need really long battery life

7) Disable segments in all of your activities

I find that with these things set I still get a very usable watch that takes advantage of what I need from it as far as tracking workouts, activity and recovery without using it as a smart watch extension of my phone.  I never really got the point of using a Bluetooth watch for that anyways since it needs your phone to be within 30 to 40 feet to transmit the data .. I'd rather just check my phone and see it on a bigger screen. 

Like I said, there are users having more issues than me. Maybe I'm lucky but I've had none of the issues you mentioned.

For a brief time I thought the GPS accuracy had gone down, but not by as much as the posted examples and it's not an issue for me in 12.27, which is the FW the OP asked about. GNSS signal lock being slow has also been an issue, so there was definitely something going on, but it's almost back to out-of-the-box speeds for me and the improvement seems to have been gradual, partially or completely regardless of firmware updates following the issue. The only lasting GNSS issue is 'Track Run' being broken, which I mentioned, and is impairing advertised/expected functionality. Garmin is aware of that issue, so hopefully a fix is coming soon.

My watch hasn't been stuck syncing on 12.27 (Android/Pixel user here) and I've never raised my wrist to find the watch with an unexpectedly flat battery. I tend to charge my watch when it reaches 15-20% for the sake of battery health in the long run and have never run out thus far.

10.10 is probably too old.

I believe the benefits from new features far outweigh the bugs.

I am running the 13.15 beta (and was also running the earlier Beta's) I don't have any issues that stop life , there may have been some irritants but the majority of the large issues seem to be sorted now (well for my use case anyway). If you are interested here are some tests I did earlier today on some of the bug fixes and new features.

https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-955-series/313702/beta-13-15---in-the-name-of-science-gap-hotkey-to-watch-face-and-gps-wrist-power-test